The present invention relates generally to the formation of tungsten metal films on substrates. More particularly, it relates to the selective formation of tungsten metal films on conductive and semiconductive surfaces of a patterned substrate.
There is a continuous trend in the development of solid state circuitry to reduce the dimensions of devices in integrated circuits and in this way increase the speed of operation. As the dimensions of devices on substrates such as silicon wafers continue to shrink, the opening in insulating layers between conductors and semiconductors becomes smaller and the lateral dimensions approach the dimension normal to the silicon surface, thus requiring steep sides to accommodate close packing and achieve the high density required. Coverage of the steep sides by the succeeding conductor becomes difficult and the recessed surface makes lithography for subsequent layers difficult.
A need is recognized for improvement in formation of conductive films on these close packed silicon wafers. A method of avoiding this problem would be to grow a metal selectively on the conductor and semiconductor surfaces. Chemical vapor deposition of tungsten on such surfaces would be a suitable method for accomplishing this goal. The reaction of tungsten hexafluoride with silicon is inherently selective but self limiting because the deposited tungsten film eventually covers all of the silicon, leaving a film of only 100 to 400 angstroms of tungsten. To deposit thicker films of tungsten one must use hydrogen to reduce tungsten hexafluoride and means must be found to prevent tungsten deposition where not desired.
Preventing the growth of tungsten on insulator surfaces has been a problem when attempting to grow thick films of tungsten selectively on conductor and semiconductor surfaces, as disclosed by Saraswat et al. in Selective CVD of Tungsten for VLSI Technology, Stanford University, May 1984. Where chemical vapor deposition methods are employed, the problem relates particularly to the nucleation and growth of tungsten on the insulator surfaces after approximately 1000 angstroms of tungsten growth on the conductor surfaces. The selectivity of tungsten vapor deposition has been found to be dependent upon many process conditions including temperature, pressure and reactant concentration. It is desirable to enhance the selectivity of tungsten vapor deposition under all process conditions.